Firestore Service Creation

Firestore Service Creation

Use this skill when creating services that interact with Firestore using async operations.

For comprehensive coding guidelines, see AGENTS.md in the repository root.

Service Structure

Create services in app/​services/ with the following structure:

"""
Resource service with async Firestore operations.
"""

from datetime import UTC, datetime
from typing import TYPE_CHECKING

from google.cloud import firestore

from app.core.firebase import get_async_firestore_client
from app.exceptions import ResourceAlreadyExistsError, ResourceNotFoundError
from app.middleware import log_audit_event
from app.models.resource import RESOURCE_COLLECTION, Resource, ResourceCreate, ResourceUpdate
# Note: Models are organized in subdirectories:
# - app/​models/​resource/​requests.py (ResourceCreate, ResourceUpdate)
# - app/​models/​resource/​responses.py (Resource, RESOURCE_COLLECTION)

if TYPE_CHECKING:
    from google.cloud.firestore import AsyncClient, AsyncDocumentReference, AsyncTransaction


class ResourceService:
    """
    Service for resource CRUD operations using async Firestore.
    """

    def __init__(self) -> None:
        self.collection_name = RESOURCE_COLLECTION

    def _get_client(self) -> AsyncClient:
        return get_async_firestore_client()

Transactional Operations

Use @firestore.async_transactional for atomic operations. Define transaction methods as static:

@staticmethod
@firestore.async_transactional
async def _create_in_transaction(  # pragma: no cover
    transaction: AsyncTransaction,
    doc_ref: AsyncDocumentReference,
    data: dict,
) -> None:
    # Tested via E2E tests with Firebase emulators; unit tests mock this method
    snapshot = await doc_ref.get(transaction=transaction)
    if snapshot.exists:
        raise ResourceAlreadyExistsError("Resource already exists")
    transaction.set(doc_ref, data)

CRUD Operations

Create

async def create_resource(self, user_id: str, resource_data: ResourceCreate) -> Resource:
    """
    Create a new resource for the given user.
    """
    client = self._get_client()
    doc_ref = client.collection(self.collection_name).document(user_id)

    now = datetime.now(UTC)
    resource_dict = {
        "id": user_id,
        **resource_data.model_dump(),
        "created_at": now,
        "updated_at": now,
    }

    transaction = client.transaction()
    await self._create_in_transaction(transaction, doc_ref, resource_dict)

    log_audit_event("create", user_id, "resource", user_id, "success")

    return Resource(**resource_dict)

Read

async def get_resource(self, user_id: str) -> Resource:
    """
    Get resource by user ID.

    Raises:
        ResourceNotFoundError: If resource does not exist.
    """
    client = self._get_client()
    doc_ref = client.collection(self.collection_name).document(user_id)
    snapshot = await doc_ref.get()

    if not snapshot.exists:
        raise ResourceNotFoundError("Resource not found")

    data = snapshot.to_dict()
    if not data:
        raise ResourceNotFoundError("Resource not found")

    return Resource(**data)

Update

Use transactions to ensure atomicity and return merged data:

@staticmethod
@firestore.async_transactional
async def _update_in_transaction(  # pragma: no cover
    transaction: AsyncTransaction,
    doc_ref: AsyncDocumentReference,
    updates: dict,
) -> dict | None:
    # Tested via E2E tests with Firebase emulators; unit tests mock this method
    snapshot = await doc_ref.get(transaction=transaction)
    if not snapshot.exists:
        return None
    existing_data = snapshot.to_dict() or {}
    transaction.update(doc_ref, updates)
    return {**existing_data, **updates}


async def update_resource(self, user_id: str, resource_data: ResourceUpdate) -> Resource:
    """
    Update an existing resource.

    Raises:
        ResourceNotFoundError: If resource does not exist.
    """
    client = self._get_client()
    doc_ref = client.collection(self.collection_name).document(user_id)

    update_dict = {k: v for k, v in resource_data.model_dump(exclude_unset=True).items() if v is not None}

    if not update_dict:
        return await self.get_resource(user_id)

    update_dict["updated_at"] = datetime.now(UTC)

    transaction = client.transaction()
    merged_data = await self._update_in_transaction(transaction, doc_ref, update_dict)

    if merged_data is None:
        raise ResourceNotFoundError("Resource not found")

    log_audit_event("update", user_id, "resource", user_id, "success")

    return Resource(**merged_data)

Delete

@staticmethod
@firestore.async_transactional
async def _delete_in_transaction(  # pragma: no cover
    transaction: AsyncTransaction,
    doc_ref: AsyncDocumentReference,
) -> dict | None:
    # Tested via E2E tests with Firebase emulators; unit tests mock this method
    snapshot = await doc_ref.get(transaction=transaction)
    if not snapshot.exists:
        return None
    data = snapshot.to_dict()
    transaction.delete(doc_ref)
    return data


async def delete_resource(self, user_id: str) -> Resource:
    """
    Delete a resource by user ID.

    Raises:
        ResourceNotFoundError: If resource does not exist.
    """
    client = self._get_client()
    doc_ref = client.collection(self.collection_name).document(user_id)

    transaction = client.transaction()
    deleted_data = await self._delete_in_transaction(transaction, doc_ref)

    if deleted_data is None:
        raise ResourceNotFoundError("Resource not found")

    log_audit_event("delete", user_id, "resource", user_id, "success")

    return Resource(**deleted_data)

Audit Logging

Use log_audit_event() from app.middleware for security-relevant operations:

from app.middleware import log_audit_event

# After successful operation
log_audit_event("create", user_id, "resource", resource_id, "success")
log_audit_event("update", user_id, "resource", resource_id, "success")
log_audit_event("delete", user_id, "resource", resource_id, "success", details={"reason": "user_request"})

Dependency Registration

Register the service in app/​dependencies.py:

from typing import Annotated
from fastapi import Depends
from app.services.resource import ResourceService


def get_resource_service() -> ResourceService:
    """
    Dependency provider for ResourceService.
    """
    return ResourceService()


ResourceServiceDep = Annotated[ResourceService, Depends(get_resource_service)]

Collection Constants

Define collection names in the response model file:

# app/​models/​resource/​responses.py
RESOURCE_COLLECTION = "resources"

Import in service:

from app.models.resource import RESOURCE_COLLECTION

Type Hints

Use TYPE_CHECKING for Firestore types to avoid import issues:

from typing import TYPE_CHECKING

if TYPE_CHECKING:
    from google.cloud.firestore import AsyncClient, AsyncDocumentReference, AsyncTransaction

Testing

Transaction methods are tested via E2E tests with Firebase emulators. Add # pragma: no cover comment and explanatory note:

@staticmethod
@firestore.async_transactional
async def _create_in_transaction(  # pragma: no cover
    transaction: AsyncTransaction,
    doc_ref: AsyncDocumentReference,
    data: dict,
) -> None:
    # Tested via E2E tests with Firebase emulators; unit tests mock this method
    ...

Unit tests mock the transactional methods or the service itself:

from unittest.mock import AsyncMock

mock_service = AsyncMock(spec=ResourceService)
mock_service.create_resource.return_value = make_resource()